1. Technical Field
The present disclosure relates to Phase-Locked Loop (PLL) based frequency synthesizers within Frequency Modulation (FM) receivers.
2. Background Information
A miniaturized and integrated FM (Frequency Modulation) radio transceiver generally involves the use of a frequency synthesizer. The frequency synthesizer typically includes a Phase-Locked Loop (PLL) and therefore may be referred to as a PLL-based frequency synthesizer. If, for example, an integrated FM radio transceiver is receiving a radio transmission, then the PLL-based frequency synthesizer within the transceiver generates a Local Oscillator (LO) signal. The LO signal is supplied to a mixer that is part of a demodulator of an FM receiver portion of the transceiver. If, on the other hand, the FM radio transceiver is transmitting a radio transmission, then the same PLL-based frequency synthesizer is used to output an FM modulated signal. The FM modulated signal is then amplified and supplied to the antenna for transmission. Of various requirements imposed on the design of such a PLL-based frequency synthesizer within an FM receiver, there are two requirements of particular importance: 1) In-band residual FM, and 2) Out-of-band SSB (Single Side Band) phase noise.
Table 1 below sets forth an example of requirements that may be imposed on a LO signal output by a PLL-based frequency synthesizer within an FM transceiver.
TABLE 1SpecificationCommentsMaxUnitsIn-Band Residual FM Mono300 Hz-5 KHz19HzrmsIn-Band Residual FM Stereo L-R33 KHz-43 KHz67HzrmsIn-Band Residual FM Stereo RDS55 KHz-59 KHz67HzrmsOut-of-Band SSB Phase Noiseat 200 KHz−112dBc/Hzat 400 KHz−124dBc/Hzat >500 KHz−126dBc/Hz
FIG. 1 (Prior Art) is a diagram that shows a 59 KHz wide portion of the FM band above an FM carrier frequency. For each FM broadcast radio station, there is such a 59 KHz wide portion above the FM carrier, and one such 59 KHz wide portion below the FM carrier. Generally speaking, in-band residual FM is found by integrating phase noise in the LO signal over one of the particular frequency range portions of FIG. 1. For example, in-band residual FM monotone is a measure of SSB phase noise in the monotone frequency range 1 of FIG. 1. In-band residual FM monotone (300 Hz to 5 KHz) can be determined by integrating SSB (Single Side Band) phase noise over the 300 Hz to 5 KHz range in accordance with Equation (1) below, where L(f) is measured PLL phase noise and has units of dBc/Hz:
                              FMres          Mono                =                              2            ·                                          ∫                                  300                  ⁢                                                                          ⁢                  Hz                                                  5000                  ⁢                                                                          ⁢                  Hz                                            ⁢                                                                    f                    2                                    ·                                      10                                                                  L                        ⁡                                                  (                          f                          )                                                                    10                                                                      ⁢                                  ⅆ                  f                                                                                        Eq        .                                  ⁢                  (          1          )                    
In-band residual FM stereo is a measure of SSB phase noise in the stereo frequency range 2 of FIG. 1. In-band residual FM stereo can be determined by integrating SSB phase noise over the 33 KHz to 43 KHz frequency range in accordance with Equation (2) below:
                              FMres          Stereo_LR                =                              2            ·                                          ∫                                  33000                  ⁢                                                                          ⁢                  Hz                                                  43000                  ⁢                                                                          ⁢                  Hz                                            ⁢                                                                    f                    2                                    ·                                      10                                                                  L                        ⁡                                                  (                          f                          )                                                                    10                                                                      ⁢                                  ⅆ                  f                                                                                        Eq        .                                  ⁢                  (          2          )                    
In-band residual FM RDS (Radio Data System) is a measure of phase noise in the RDS/RBDS frequency range 3 of FIG. 1. In-band residual FM RDS can be determined by integrating SSB phase noise over the 55 KHz to 59 KHz frequency range in accordance with Equation (3) below:
                              FMres          RDS                =                              2            ·                                          ∫                                  55000                  ⁢                                                                          ⁢                  Hz                                                  59000                  ⁢                                                                          ⁢                  Hz                                            ⁢                                                                    f                    2                                    ·                                      10                                                                  L                        ⁡                                                  (                          f                          )                                                                    10                                                                      ⁢                                  ⅆ                  f                                                                                        Eq        .                                  ⁢                  (          3          )                    
The quality of audio output by an FM receiver is generally limited by the in-band residual FM. When no jammer is present, residual FM is usually the limiting performance parameter and determines the effective audio Signal-to-Noise Ratio (SNR) and hence audio quality provided that the Carrier-to-Noise Ratio (CNR) is high. When a jammer is present, however, then the quality of the audio output by the FM receiver is generally limited by out-of-band SSB phase noise. The jammer and the wanted signal being received are FM demodulated, giving rise to audio distortion due to reciprocal mixing. This audio distortion dominates the deleterious effect of the in-band residual FM phase noise. In one example, a jammer is a signal that is of such a frequency and power that it reciprocally mixes with phase noise of the LO signal in the receiver in such a way that substantial in-band signal-to-noise degradation results. The transmitted FM signal of an adjacent FM radio channel may be an example of one such jammer.
In a conventional miniaturized and integrated FM receiver involving a PLL-based frequency synthesizer, the various components of the PLL-based frequency synthesizer are generally sized and tuned to achieve acceptable performance under both the no jammer condition and the jammer condition.